In general, a feeding voltage and a voltage for generating a ringer signal (hereinafter, referred to as a ringer voltage) for telephone lines is each generated on the basis of a relatively high-level direct-current voltage of 24 V or the like by means of a booster circuit. The ringer signal is an alternating-current signal of 20 Hz and 200 Vp-p (peak to peak), i.e., approximately 70 Vrms (root mean square), and is a signal which periodically repeats a time frame consisting of an on duration of 1 second and an off duration of 2 seconds. When the ringer signal has been inputted to a telephone connected to a telephone line therethrough, the telephone rings a bell or the like for signaling an incoming call.
Common apparatuses capable of connecting to at least one telephone, such as copying machines, routers and private branch exchanges (PBXs), are each configured to, in a power saving mode, supply only a low-level direct-current voltage of 3.3 V or the like, which is supplied to circuits for a control system, such as a circuit for a CPU, and suspend supply of high-level direct-current voltages. As a result, electric power consumed by these apparatuses can be reduced. Under the state where such an apparatus feeds electric power to at least one telephone connected thereto, generally, the apparatus generates a feeding voltage and a ringer voltage from a high-level direct-current voltage of −24 V or −48 V. Consequently, under the state where at least one telephone is fed with electric power from the apparatus, when the apparatus has entered the power saving mode, the at least one telephone cannot be supplied with the feeding voltage and the ringer voltage which are generated from the high-level direct-current voltage. Thus, when the apparatus is in the power saving mode, the at least one telephone is likely to be unusable. For this reason, it is desirable that such an apparatus that feeds electric power to at least one telephone connected thereto generates the feeding voltage and the ringer voltage from the low-level direct-current voltage of 3.3 V or the like.
In the case where the feeding voltage and the ringer voltage are generated from the low-level direct-current voltage of 3.3 V or the like, the boosting ratio of a booster circuit becomes large. Consequently, the feeding-voltage level and the ringer-voltage level are likely to fluctuate to a great degree because of the variation of accuracy with respect to a transformer and the like used in the booster circuit. The ringer voltage is a high-level voltage of 200 Vp-p or the like, and thus, in the case where the voltage level of the ringer signal becomes larger than a setting value because of the variation thereof, it is likely to be difficult to maintain the safety of related circuits. In contrast, in the case where the voltage level of the ringer signal becomes smaller than the setting value, sometimes, there occur inconvenient situations, such as a situation in which a bell for signaling an incoming call included in each of the at least one telephone does not ring even though supplied with a ringer signal, so that it is likely to be difficult for the at least one telephone to operate normally.
Meanwhile, there exists a circuit for adjusting a feeding-voltage level and a ringer-voltage level in such a way that a voltage signal obtained by switching an output voltage outputted by a direct-current power source is boosted by a booster/rectifier circuit; an output voltage outputted by the booster/rectifier circuit is monitored; and the feeding-voltage level and the ringer-voltage level are adjusted while changing the timing of the switching operation on the basis of the monitor result (for example, refer to Japanese Unexamined Patent Application Publication No. 6-351050). Further, there exists a circuit for preventing a rush current which arises when a ringer signal transits from an ON state (a ringing transmission state) to an OFF state (a silence transmission state), and which arises when the ringer signal transits from the silence transmission state to the ringing transmission state (for example, refer to Japanese Unexamined Patent Application Publication No. 9-162975).